Implement WebDAV VFS integration: dav-server 0.11 compatible

- Add webdav.rs module: VfsDavFs, VfsDavFile, VfsDavMetaData
- Implement DavFileSystem + Clone for GuardedFileSystem blanket impl
- Add clone_boxed to VfsBackend trait (required for Sync)
- Update CLI webdav.rs to use VFS instead of SQLite
- Add bytes dependency
- All 155 tests pass

markbase-core/src/vfs/local_fs.rs

@@ -61,6 +61,10 @@ impl VfsFile for LocalFile {
 }
 
 impl VfsBackend for LocalFs {
+    fn clone_boxed(&self) -> Box<dyn VfsBackend> {
+        Box::new(Self {})
+    }
+
     fn read_dir(&self, path: &Path) -> Result<Vec<VfsDirEntry>, VfsError> {
         let dir = fs::read_dir(path).map_err(|e| util::map_io_error(path, e))?;
 

markbase-core/src/vfs/mod.rs

@@ -115,7 +115,10 @@ pub trait VfsFile {
 }
 
 /// VFS 后端 trait（所有文件系统操作）
-pub trait VfsBackend: Send {
+pub trait VfsBackend: Send + Sync {
+    /// Clone boxed
+    fn clone_boxed(&self) -> Box<dyn VfsBackend>;
+
     /// 读取目录内容
     fn read_dir(&self, path: &Path) -> Result<Vec<VfsDirEntry>, VfsError>;
 

markbase-core/src/vfs/s3_fs.rs

@@ -193,6 +193,13 @@ impl S3Vfs {
 }
 
 impl VfsBackend for S3Vfs {
+    fn clone_boxed(&self) -> Box<dyn VfsBackend> {
+        Box::new(Self {
+            bucket: self.bucket.clone(),
+            credentials: self.credentials.clone(),
+        })
+    }
+
     fn read_dir(&self, path: &Path) -> Result<Vec<VfsDirEntry>, VfsError> {
         let prefix = Self::dir_key(path);
         let list = self.list_objects(&prefix)?;